Protostome Animals.

Protostome Animals

Chapter 31 Opening Roadmap.
© 2017 Pearson Education, Inc.

Introduction Protostome animals are spectacularly diverse and abundant
Of the 30 animal phyla described to date, 22 are protostomes

Table 31.1 Key Lineages of Lophotrochozoans.

Introduction Some protostome phyla have little diversity, such as the phylum Priapula, which contains only 16 named species On the other end of the spectrum, over 85,000 mollusks have been named thus far, and about 1.2 million arthropod species, of which about a million are insects Scientists estimate that the actual number of arthropod species may be over 10 million

Major protostome phyla
Figure 31.1 Major protostome phyla Major non-protostome phyla Arthropoda Chelicerates Nematoda Crustaceans Myriapods Mollusca Insects Platyhelminthes Annelida Chordata Figure 31.1 The Relative Diversity of Animal Lineages. Echinodermata Cnidaria Porifera Other invertebrates © 2017 Pearson Education, Inc.

Introduction Protostomes fill very important ecological roles
As a group, they live in virtually every aquatic and terrestrial habitat They can be detritivores, herbivores, or carnivores The ecological importance of protostomes extends to human health

Introduction Importance of protostomes:
Major direct source of food for humans Provide ecosystem services Some damage crops Some produce materials such as silk and pearls Cause or transmit human diseases and are parasites Include two of the most important model organisms Fruit fly Drosophila melanogaster and roundworm Caenorhabditis elegans

What Is a Protostome? The exact order of branches is still under debate, but researchers agree that the sponges (phylum Porifera), comb jellies (phylum Ctenophora), and jellyfish and corals (phylum Cnidaria) were the first lineages to emerge during the early radiation of animals Bilaterian ancestor was likely bilaterally symmetric and triploblastic, which gave rise to an array of body plans among bilaterians during the Cambrian explosion

What Is a Protostome? Biologists traditionally distinguished the protostomes from deuterostomes using developmental characteristics: Embryonic development of the mouth before the anus during gastrulation Inability of isolated early embryonic cells to develop into a complete embryo The formation of a coelom by the splitting of blocks of mesodermal cells

What Is a Protostome? Newer view is that there are many exceptions to the “protostome versus deuterostome” developmental patterns For example, the anus develops before the mouth in some protostomes Some similar developmental characteristics evolved independently in different lineages Convergent evolution Other characteristics thought to be synapomorphies were lost in some groups

What Is a Protostome? Recent DNA sequence data support two major subgroups within the protostomes: Lophotrochozoa include mollusks and annelid worms Ecdysozoa include the nematodes and arthropods

Porifera Ctenophora ANIMALS Cnidaria LOPHOTROCHOZOA Rotifera
Figure 31.2 Porifera Ctenophora ANIMALS Cnidaria LOPHOTROCHOZOA Rotifera Platyhelminthes Annelida PROTOSTOMES Mollusca ECDYSOZOA Nematoda Figure 31.2 Protostomes Are a Monophylatic Group Comprising Two Major Lineages. Tardigrada Onychophora BILATERIANS Arthropoda Echinodermata DEUTEROSTOMES Chordata © 2017 Pearson Education, Inc.

The Water-to-Land Transition
Fossils indicate that protostome lineages originated in the ocean Like land plants and fungi, protostomes made the transition from aquatic to terrestrial environments However, protostomes made this transition multiple times as they diversified

Cnidaria LOPHOTROCHOZOA Rotifera Platyhelminthes Annelida Mollusca
Figure 31.3 Cnidaria LOPHOTROCHOZOA Rotifera Platyhelminthes Annelida Mollusca All aquatic ECDYSOZOA Nematoda Tardigrada Onychophora Arthropoda Echinodermata Chordata Some terrestrial All terrestrial Figure 31.3 The Water-to-Land Transition Occurred Several Times in the Protostomes. © 2017 Pearson Education, Inc.

Water-to-land transitions are important because they open up entirely new habitats and new resources to exploit To make the transition to land, new adaptations must allow protostomes to Exchange gases Avoid drying out Hold up their bodies under their own weight Terrestrial protostomes have evolved many solutions to these challenges

Roundworms and earthworms have a high surface-area-to-volume ratio This increases the efficiency of gas exchange across their body surface in their moist environments Some terrestrial arthropods and mollusks have gills or other respiratory structures located inside the body This minimizes water loss when moving onto land

Insects evolved a waxy layer to minimize water loss from the body surface If the environment dries, openings to respiratory passages can be closed Desiccation-resistant eggs evolved repeatedly in populations that made the transition to life on land Insect eggs have a thick membrane that keeps in moisture Snail and slug eggs have a thick shell that retains water

Modular Body Plans Morphological and physiological diversification has a genetic basis Until recently, biologists wrongly assumed that very different organisms required very different genetic instructions Multicellular animals have a common tool kit of genes that establish the animal body plan during development Hox genes are an important part of this tool kit Expressing the genes in the tool kit at different times and places during development can lead to dramatic differences

Modular Body Plans Diversification of animal body plans can occur by the generation of new genes over time However, changing the expression pattern of existing genes likely had an even larger impact on animal body plan diversification When comparing diverse organisms within a lineage, genetically based modularity is evident A small set of elements can be reused and rearranged to produce a large variety of outcomes

What Is a Lophotrochozoan?
The lophotrochozoans are a monophyletic group—all descendants of a common ancestor The 13 phyla within the Lophotrochozoa include the rotifers, flatworms, annelids, and mollusks Two morphological traits occur in some, but not all, members: Feeding structure called a lophophore, which is found in three phyla A type of larva called a trochophore, which is common to many of the phyla

A lophophore is a specialized structure that rings the mouth of these animals and functions in suspension feeding Trochophores are a type of larva common to several phyla of lophotrochozoa Trochophore larvae have a ring of cilia around their middle that functions in sweeping and, sometimes, in feeding

(a) Lophophores function in suspension feeding in adults.
Figure 31.4 (a) Lophophores function in suspension feeding in adults. (b) Trochophore larvae swim and may feed. Food particles Food particles Water current Anus Mouth surrounded by the lophophore (ring of ciliated tentacles) Mouth Cilia used in locomotion and feeding Figure 31.4 Lophotrochozoans Are Named after Two Traits. Anus Gut 0.1 mm 0.1 mm © 2017 Pearson Education, Inc.

Trochophore larvae occur in animals that undergo indirect development, live in different habitats, and eat different foods Recent analyses suggest that trochophore larva originated early in the evolution of lophotrochozoans and later evolved into different larval types in some groups However, larvae are not unique to lophotrochozoans—even sponges have larvae

Spiral pattern of cleavage in embryos is a synapomorphy for this monophyletic group When cells divide at oblique angles to each other during early embryogenesis, a spiraling pattern of cells in the blastula results In contrast, in radial cleavage, cells divide at right angles to each other Spiral cleavage has been highly conserved in some lophotrochozoan phyla, but has been modified or lost in others

(a) Spiral cleavage is unique to lophotrochozoans.
Figure 31.5 (a) Spiral cleavage is unique to lophotrochozoans. (b) Radial cleavage, for comparison Figure 31.5 Spiral Cleavage Is a Synapomorphy for the Lophotrochozoans. © 2017 Pearson Education, Inc.

In sum, you could be confident that an animal belongs to the Lophotrochozoa if you observe any of the following: A lophophore A trochophore Spiral cleavage However, not all lophotrochozoans possess all three of these characteristics

Many lophotrochozoan phyla include species that have long, thin, tubelike bodies that lack limbs; they are worms with a basic tube-within-a-tube design The outside tube is the skin, which is derived from ectoderm The inside tube is the gut, which is derived from endoderm Muscles and organs derived from mesoderm are located between the two tubes Worms may or may not have a coelom

Table 31.1 Key Lineages of Lophotrochozoans.

What Is a Flatworm? Flatworms are named for the broad, flattened shape of their bodies They lack a coelom and structures specialized for gas exchange and circulation of oxygen and nutrients Molecular data confirms the placement of flatworms within the Lophotrochozoa

What Is a Flatworm? Biologists hypothesize that the flattened body is an adaptation that provides a large surface area for gas exchange This flat body plan allows nutrients and gases to diffuse efficiently to all of the cells inside the animal with minimal expenditure in complex internal structures This requires flatworms to live in an aquatic or moist environment

(a) Free-living turbellarian (b) Parasitic fluke
Figure 31.6 (a) Free-living turbellarian Eye spots Gut Pharynx Mouth 0.5 mm Male Female (b) Parasitic fluke Figure 31.6 Wormlike Lophotrochozoans Are Diverse. © 2017 Pearson Education, Inc.

What Is a Flatworm? The flatworms are a large and diverse phylum consisting of four major lineages: Turbellaria Cestoda Trematoda Monogenea

What Is a Flatworm? Turbellaria is a paraphyletic group of free-living flatworms Most are freshwater or marine Prey on protists or small animals, or scavenge dead animals Have a blind digestive tract Mouth is near middle of body’s ventral surface, at the end of a tubelike structure called the pharynx Can extend and pierce live prey or other food

What Is a Flatworm? Cestoda are strictly endoparasitic tapeworms that parasitize diverse vertebrates, absorbing nutrients by diffusion across their body wall Humans most often acquire tapeworms by eating undercooked pork, beef, or fish

What Is a Flatworm? Trematoda are endoparasitic or ectoparasitic flukes that parasitize vertebrates, arthropods, annelids, and mollusks A fluke is responsible for schistosomiasis—a serious public health issue in many tropical and subtropical nations, infecting more than 200 million people worldwide They feed by gulping host tissues and fluids, often causing blood in the host’s urine or stool, kidney failure, and other problems

What Is a Flatworm? Monogenea are tiny ectoparasites that parasitize specific tissues of particular species, usually the skin or gills of fishes Hermaphroditic

What Is a Segmented Worm?
Most annelids have a coelom, a fully developed digestive tract with a mouth and an anus, and a segmented body Traditional view grouped annelids and arthropods together in the same clade due to their segmentation

Molecular data suggests that segmentation arose independently in these groups But, some of the same developmental tool-kit genes are involved in segmentation in both phyla Convergent evolution at the morphological level Homology at the genetic level

Common ancestor of annelids had a key synapomorphy in addition to segmentation: Numerous, bristle-like extensions called chaetae that extend from lobe-like appendages called parapodia These appendages appear very different from appendages of other bilaterian phyla But homologous expression patterns of genes such as Dll, in appendages of diverse phyla, suggest a common origin of the genetic tool kit for appendages

(d) Terrestrial earthworm
Figure 31.6 (c) Marine polychaete 1 cm Bristle-like chaetae on parapodia (d) Terrestrial earthworm Figure 31.6 Wormlike Lophotrochozoans Are Diverse. © 2017 Pearson Education, Inc.

Three main groups: Polychaeta Oligochaeta Hirudinea

Polychaeta Diverse worms that live in a wide variety of marine habitats Polychaetes that are highly mobile often have large parapodia and chaetae More sedentary polychaetes have reduced parapodia and smaller, but still numerous, chaetae

Oligochaeta Earthworms and other oligochaetes Deposit feeding in soils Ecosystem services include: Tunnels that are important in aerating soil Feces contribute large amounts of organic matter

Hirudinea Leeches Ectoparasites that attach themselves to fish, humans, or other hosts and suck blood and other body fluids Nonparasitic leech species are predators or scavengers

Molecular data have added complexity to this three-group categorization The Polychaeta are paraphyletic Unsegmented worms called Sipunculida and Echiura (traditionally thought to be independent phyla) are now grouped within Annelida—their ancestors were segmented, but this character was lost over time

What Is a Mollusk? Mollusks are a highly diverse monophyletic group of lophotrochozoans Characteristic body plan: Foot—a large muscle located at base of the animal, used in movement Visceral mass—the region containing most of the main internal organs and external gill Mantle—outgrowth of the body wall that covers the visceral mass, forming an enclosure called the mantle cavity

Visceral mass (internal organs and external gill) Mantle
Figure 31.7 Shell (when present) Mantle cavity Gill Radula Figure 31.7 Mollusks Have a Distinct Body Plan. Muscular foot Visceral mass (internal organs and external gill) Mantle (secretes shell) © 2017 Pearson Education, Inc.

What Is a Mollusk? Once the molluscan body plan evolved, subsequent diversification was largely driven by adaptations that allowed the mollusks to move, feed, or reproduce in novel ways Dramatic radiation of mollusks into several lineages: Chitons (mollusks with dorsal shells made of plates) Bivalves (clams and mussels) Gastropods (slugs and snails) Cephalopods (squid and octopuses)

Table 31.2 Key Lineages of Mollusks.

The Foot Is a Muscular Hydrostat
Snails and chitons have a large, muscular foot at the base of the body that works as a type of hydrostatic skeleton called a muscular hydrostat Waves of muscle contractions sweep backward or forward along the length of the foot, allowing individuals to crawl along a surface In bivalves, the foot is modified as a digging appendage In cephalopods, the foot is modified to form tentacles for crawling and grasping

Stripes are waves of muscle contraction Figure 31.8
Figure 31.8 How Snails Walk on One Foot. © 2017 Pearson Education, Inc.

The Visceral Mass Separates Internal Organs from the Hydrostatic Skeleton
The visceral mass is the region in all mollusks where organs and surrounding fluids are located, separate from the muscular foot Separation of the foot from the visceral mass may have enabled greater diversification of both features across the phylum

The coelom is highly reduced in most mollusks, functioning mostly in reproduction and excretion of wastes Organs occupy a different type of body cavity called a hemocoel, where body fluids bathe the organs directly in an open circulatory system Different from a coelom because it is not lined in mesoderm and has a distinct developmental origin

At the anterior end of the visceral mass, the mouth has a feeding structure called a radula, which functions like a rasp or file Moves the radula back and forth over the food source, causing the many sharp plates to scrape material so that it can be ingested Radula probably evolved early in molluscan evolution Lost in the bivalves, which acquire food by suspension feeding

50 µm Rasp-like radula scrapes off pieces of food Figure 31.9
Figure 31.9 The Radula Is Unique to Mollusks. © 2017 Pearson Education, Inc.

The Mantle Has Diverse Functions
In many species, the mantle secretes a shell made of calcium carbonate Some mollusk species have a shell with one, two, or eight parts, called valves; others have no shell at all In bivalves, the protective shell is hinged and closes Many marine and terrestrial snails can retract into their shells when they are attacked or when their tissues begin to dry out

A protective shell may have been an important adaptation But, shells made of calcium carbonate are heavy Thus, there is a trade-off between protection and mobility Mollusks with thick shells are constrained to aquatic habitats, where buoyant forces help support the load The largest mollusks have highly reduced shells or none at all Terrestrial mollusks have thin shells or none at all

Numerous adaptations of the mantle allow diverse functions other than secreting shells In some terrestrial snails, the mantle forms an internal lung In bivalves and cephalopods, the mantle is lined with muscle and forms tubes called siphons

Numerous adaptations of the mantle allow diverse functions other than secreting shells In clams, two siphons extend into the water column above, controlling incurrent and excurrent water flow over the gills In cephalopods, the mantle cavity fills with water and then mantle muscles contract, forcing a stream of water out of a single siphon The force of the expelled water provides a form of locomotion called jet propulsion

mantle fills with water
Figure 31.10 Cavity enclosed by mantle fills with water Water is forced out through siphon; animal moves Figure Jet Propulsion in Mollusks. © 2017 Pearson Education, Inc.

What Is an Ecdysozoan? Unlike lophotrochozoans that grow continuously and incrementally, ecdysozoans grow intermittently by molting Shedding of the soft cuticle or hard exoskeleton Once the animal molts, fluid causes the body to expand—then a new, larger cuticle or exoskeleton forms Ecdysozoans undergo a succession of molts as they grow Sometimes dramatic morphological transformations occur during these molts

What Is an Ecdysozoan? The cuticle and exoskeleton
Protect these animals from predators Provide an effective structure for muscle attachment During molting, the animal’s soft body is exposed and vulnerable

(a) Lophotrochozoans grow incrementally. (b) Ecdysozoans grow
Figure 31.11 (a) Lophotrochozoans grow incrementally. (b) Ecdysozoans grow by molting. Figure Lophotrochozoans and Ecdysozoans Differ in Their Mechanism of Growth. Growth bands © 2017 Pearson Education, Inc.

Table 31.3 Key Lineages of Mollusks.

What Is a Roundworm? Species in the phylum Nematoda are commonly called roundworms or nematodes Unsegmented worms with a pseudocoelom, a tube-within-a-tube body plan, no appendages, and an elastic cuticle that is molted during growth Lack specialized systems for exchanging gases and circulating nutrients Gas exchange occurs across the body wall, and nutrients move by diffusion from the gut to other parts of the body

What Is a Roundworm? Roundworms play an important role in their ecosystems Feed on a wide variety of materials including bacteria, archaea, fungi, plant roots, small protists, and detritus In most species, the mouthparts are structured in a way that increases the efficiency of feeding on a particular type of organism or material Most roundworms are free-living Some species parasitize animals, including humans

What Are Tardigrades and Velvet Worms?
Tardigrada and Onychophora are of special interest due to their close relationship to arthropods Similar to arthropods in having a segmented body and segmented limbs Unlike arthropods, their cuticle is not hardened as an exoskeleton, and their limbs are not jointed

What Are Tardigrades and Velvet Worms?
The tardigrades, or “water bears,” are microscopic animals that live in diverse marine, freshwater, and terrestrial environments, such as in the film of water that covers plants in moist habitats The onychophorans, or velvet worms, are small, caterpillar-like organisms that live in moist leaf litter and prey on small invertebrates

What Is an Arthropod? In terms of duration in the fossil record, species diversity, and abundance of individuals, arthropods are the most important phylum within the Ecdysozoa Appear in the fossil record over 520 million years ago Most abundant animals observed in both aquatic and terrestrial environments Over a million living species have been described

Arthropod Body Plan Arthropods are defined by three key features:
Segmented body organized into prominent regions, or tagmata (singular: tagma) Body of grasshopper and other insects is divided into a head, thorax, and abdomen Body of a spider or a crayfish is divided into the cephalothorax (“head-chest”) and abdomen

Arthropod Body Plan Arthropods are defined by three key features:
Exoskeleton made primarily of the polysaccharide chitin, strengthened by calcium carbonate (CaCO3) in crustaceans Jointed appendages, which enable the rigid body to move

(a) Insects have three tagmata.
Figure 31.12 (a) Insects have three tagmata. Tagmata Head Thorax Abdomen Jointed limbs Segmented body Exoskeleton (covers body) (b) Spiders have two tagmata. Tagmata Abdomen Figure Arthropods Have a Specialized Body Plan. Cephalothorax Chelicerae Pedipalp © 2017 Pearson Education, Inc.

Why Has the Arthropod Body Plan Been So Successful?
Like mollusks, arthropod bodies are modular in evolutionary terms Studies of Hox genes and other tool-kit genes show that Small changes in the timing and location of gene expression can result in novel shapes and sizes Variation in gene expression, combined with ecological opportunity through natural selection: Can result in the diversification of arthropod body segments and appendages

Primary functions: Sensory Feeding Defense Walking Swimming
Figure 31.13 Primary functions: Sensory Feeding Defense Walking Swimming Figure Arthropods Have a Modular Body Plan. © 2017 Pearson Education, Inc.

Origin of the Wing Insects were the first animals on Earth that had wings and could fly Hypotheses about the origin of wings vary, and include Independent origin hypothesis Wings arose as outgrowths from the thorax independent from the legs Gill co-option hypothesis Wings arose from gill-like projections on the branched legs of a wingless ancestor

(a) Independent origin hypothesis
Figure 31.14 (a) Independent origin hypothesis Body cross- section Wing Leg Leg (b) Gill co-option hypothesis Figure Alternative Hypotheses for the Origin of Wings in Insects. Gill Wing Leg Leg © 2017 Pearson Education, Inc.

Origin of the Wing The hypotheses were tested using developmental genetics Results strongly support the gill co-option hypothesis

Arthropod Diversity Arthropod taxonomy traditionally recognizes four major lineages: Myriapods Insects Crustaceans Chelicerates

Myriapods Have Long, Segmented Trunks
Myriapods have relatively simple bodies: A head region A long trunk with segments that each have one pair of legs (centipedes) or two pairs of legs (millipedes) Some species have no eyes, but others have from a few to many simple eyes clustered on the sides of the head Myriapods inhabit terrestrial environments all over the world

Insects Have Three Tagmata, Unbranched Appendages, and One Pair of Antennae
Insects have three tagmata—a head, thorax, and abdomen Three pairs of walking legs extend from the sides of the thorax Most species have one or two pairs of wings on the back of the thorax

Insects Have Three Tagmata, Unbranched Appendages, and One Pair of Antennae
Head has four pairs of mouthpart structures, one pair of slender antennae that are used to touch and smell, and a pair of compound eyes A compound eye contains many lenses, each associated with a light-sensing, columnar structure ~1 million species of insects have been named thus far, but it is certain that many more exist Insects dominate terrestrial environments Larvae of some species are common in freshwater aquatic environments

Crustaceans Have Two or Three Tagmata, Branched Appendages, and Two Pairs of Antennae
Crustaceans live primarily in aquatic environments, where they play important ecological roles: Planktonic copepods—consume phytoplankton and, in turn, are an important food source for fish and marine mammals Just a few species of crabs and some isopods are terrestrial

The segmented body of most crustaceans is divided into three tagmata: the head, thorax, and abdomen Some species have a carapace (platelike section of exoskeleton) that covers and protects head and thorax Crustacean appendages are usually branched, but some have lost their branch, becoming secondarily unbranched

Two pairs of antennae; sophisticated, compound eyes (usually on stalks) Most have 4–6 pairs of mouthparts derived from jointed appendages Many have mouthparts on head (maxillae) Some have a pair of mouthparts called mandibles that can bite or chew Many have feeding appendages on thorax segments, called maxillipeds or claws

Chelicerates Have Two Tagmata and Chelicerae
The most prominent lineages of chelicerates are terrestrial (spiders, scorpions, ticks, mites, and daddy longlegs) Some are marine (horseshoe crabs and sea spiders)

Chelicerate body consists of two tagmata: the cephalothorax (prosoma) and abdomen opisthosoma) Chelicerate tagmata are not homologous to those in other arthropods, even though they are functionally similar Cephalothorax lacks antennae, but usually has eyes

Named for a pair of clawlike appendages called chelicerae, located near the mouth Depending on the species, the chelicerae are used in feeding, defense, copulation, movement, or sensory reception Also have a pair of pedipalps located just behind the chelicerae, which may be used to manipulate food, transfer sperm, or carry out other functions

New Phylogeny of Arthropods
Relationships of lineages within the arthropods are currently undergoing a major revision Insects and myriapods have long been considered sister groups within the arthropods due to shared morphological characteristics However, recent phylogenetic studies provide strong support for the placement of the insect clade within the crustacean lineage, making the traditional concept of crustaceans paraphyletic

Figure 31.15 Phylogeny of the Major Groups of Arthropods.
ONYCHOPHORA CRUSTACEA Crabs Lobsters Shrimp Isopods Copepods Two pairs of antennae Barnacles Brine shrimp Remipedes Loss of one pair of antennae Insects MYRIAPODA Centipedes Millipedes ARTHROPODA Single pair of antennae Figure Phylogeny of the Major Groups of Arthropods. CHELICERATA Sea spiders Mites Chelicerae and pedipalps Ticks Horseshoe crabs Scorpions Spiders © 2017 Pearson Education, Inc.

Two Types of Insect Metamorphosis
In insects, the presence or absence of a larval stage defines two distinct types of metamorphosis Hemimetabolous metamorphosis (incomplete metamorphosis) Form of direct development Juveniles called nymphs look like smaller versions of the adult

(a) Aphid: Incomplete metamorphosis (hemimetabolous metamorphosis)
Figure 31.16a (a) Aphid: Incomplete metamorphosis (hemimetabolous metamorphosis) Nymphs Figure 31.16a Insect Metamorphosis Can Be Incomplete or Complete. Nymphs look like smaller versions of adults and eat the same foods. © 2017 Pearson Education, Inc.

Two Types of Insect Metamorphosis
Holometabolous metamorphosis (complete metamorphosis) There is a distinct larval stage Larvae and adults often live in different habitats and feed on different foods In holometabolous metamorphosis, larvae stop feeding and moving and secrete a protective case, becoming a pupa During pupation, the pupa’s body is completely remodeled into a new, adult form

(b) Mosquito: Complete metamorphosis (holometabolous metamorphosis)
Figure 31.16b (b) Mosquito: Complete metamorphosis (holometabolous metamorphosis) Adult Pupae Figure 31.16b Insect Metamorphosis Can Be Incomplete or Complete. Larvae Larvae look substantially different from adults and eat different foods. © 2017 Pearson Education, Inc.

What Is the Adaptive Significance of Metamorphosis?
Complete metamorphosis is 10 times more common than incomplete metamorphosis Biologists hypothesize that this is because of feeding efficiency If adults and juveniles feed on different materials in different ways, they do not compete with each other for resources

What Is the Adaptive Significance of Metamorphosis?
Another hypothesis is based on the advantages of functional specialization Specialization can lead to higher efficiency in feeding and reproduction and thus higher fitness

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